Currently, the standardization of the LTE (Long Term Evolution) system, which is a wireless communication system for mobile phones of the 3.9th generation, is almost completed. Recently, LTE-A (LTE-Advanced), which is a fourth-generation wireless communication system being a more advanced version of the LTE system, has started to be standardized.
In general, for an uplink of a mobile communication system (communication from mobile stations to a base station), mobile stations serve as transmitting stations, and thus a single-carrier method (LET employs SC-FDMA (Single Carrier Frequency Division Multiple Access)) is considered effective due to its low peak power that enables the power use efficiency of an amplifier to be maintained at a high level with a limited transmission power. Note that SC-FDMA is also called DFT-S-OFDM (Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing).
A single-carrier method such as SC-FDMA assumes a continuous spectrum arrangement. Thus, under an environment where frequency selective fading occurs due to a delay of an electric wave in a radio propagation path, part of the spectrum has low gain of the propagation path and therefore makes the received power significantly low. The energy loss remains as it is even when an adaptive equalization technique (such as frequency domain equalization) is used.
In this respect, Non-patent Document 1 listed below has disclosed a technique of performing spectral shaping based on a water filling theorem (also called WF) which can maximize the received energy.
FIG. 8 is a diagram showing the principle described in Non-patent Document 1. In the upper part of FIG. 8, the horizontal axis is frequency while the vertical axis is signal-to-noise power ratio (SNR). L1001 in Part (a) of FIG. 8 represents the propagation path gain at each frequency estimated in an uplink. In addition, in Part (b) of FIG. 8, the horizontal axis represents frequency while the vertical axis represents the transmission power at each frequency. The horizontal axes in Parts (a) and (b) of FIG. 8 are matched with each other. Under such a frequency selective fading environment, there are frequencies with high propagation path gains such as ones in a band F1001, while there are frequencies with extremely low gains such as ones in F1002. Thus, as shown by S1001 in Part (b) of FIG. 8, in the single-carrier spectrum, higher transmission powers are allocated to the band F1001 while lower transmission powers are allocated to frequencies with lower gains. Further, the received energy is maximized with the same amount of transmission power by a clipping process which allocates no power to frequencies with extremely low gains such as ones in F1002.
To utilize the received energy thus maximized, the receiving side makes detection through turbo equalization in which the transmission energy allocated based on the water filling theorem is regarded as the frequency gains. Thus, the receiving side can perform accurate detection.